The invention relates to an apparatus for applying surface pressure to advancing workpieces, such as wooden boards, boards made of wood materials or the like, the apparatus including at least one rotating pressing belt which can be pressed by means of a pressure medium against the workpiece in a pressure chamber disposed on the inside of the pressing belt and equipped with a seal.
In connection with such apparatus, it has been known to provide, above or below a workpiece passing on the upper side or on the underside, a single pressure chamber which covers approximately the entire width of the advancing workpiece. Such large pressure chambers, which are charged by a pressure medium such as compressed air, have various drawbacks. The sealing strips oriented in the direction of the pressing belt at the open side of such a pressure chamber are under considerably greater stress in the advancing direction of the workpiece than the seals oriented transversely to the direction of advancement of the workpiece. This leads to differences in abrasion and premature wear and also to losses of air. Due to the great length of the sealing strips, such a seal cannot be made of one piece. There exist one or a plurality of junctures which also lead to leaks or pressure losses. Great forces originating from the pressure medium act on the sealing strip so that they must be supported which is possible only at considerable structural expense. If there is a sudden great loss of pressure, the workpiece presses the pressing belt against the inner surfaces of the pressure chambers. This produces considerable water unless these inner surfaces are coated with a special antifriction material. The measures, taken merely to realize emergency running properties, are complicated and difficult to accomplish.